Thixotropic metal forming processes may be used to either make a raw material or fabricate a finished part. These processes are also known as thixocasting, thixomolding, thixoforming, semi-solid forming, semi-solid forging, semi-solid casting, semi-solid metal processing or rheocasting. Thixotropic metal forming is based on a discovery made by Professor M. C. Flemings during research on hot tearing undertaken at MIT in the early 1970s. Seeking to understand the magnitude of the forces involved in deforming and fragmenting dendritic growth structures, MIT researchers constructed a high-temperature viscometer. They poured model lead-tin alloys into the annular space created by two concentric cylinders and measured the forces transmitted throughout the freezing alloy when the outer cylinder was rotated.
During the course of these experiments, it was discovered that when the outer cylinder was continuously rotated the semi-solid alloy exhibited remarkably low shear strength even at relatively high fractions solidified. The unique property was attributed to a novel nondendritic, spheroidal microstructure. The MIT researchers were quick to identify several potential benefits that could result from forming processes utilizing semi-solid metal and that would differentiate these processes from conventional casting. First, and particularly significant for higher melting alloys, semi-solid metalworking afforded lower operating temperatures and reduced metal heat content (reduced enthalpy of fusion). Second, the viscous flow behavior could provide for a more laminar cavity fill than could generally be achieved with liquid alloys. This could lead to reduced gas entrainment. Third, solidification shrinkage would be reduced in direct proportion to the fraction solidified within the semi-solid alloy, which should reduce both shrinkage porosity and the tendency toward hot tearing.
Raw material for semi-solid forming part production requires the special microstructure. When semi-solid, this structure is comprised of solid particles in the form of spheroids or globules suspended in a matrix of lower melting alloy liquid. Recapturing this structure in materials heated from the solid state requires the retention of some residual microsegregation to provide differential melting between solid and liquid phases. In the disk drive actuator art, practitioners have proposed numerous incremental improvements to the actuator assembly design typified by actuator assembly 10 in FIG. 1. For instance, in U.S. Pat. No. 5,122,703, Fumihiko Takahashi et al. teach an improvement in joining coil 22 to E-block 12 that consists of fixing the two together by a hold member made of a thermal plastic resin having an elastic modulus greater than a specified value. In U.S. Pat. No. 5,148,071, Takahashi teaches the use of a nonconductive stiffening plate disposed over coil 22, with both coil and plate integrally molded (over-molded) to E-block 12. Both Takahashi inventions teach solutions to the coil-block joint flexure problem known to cause head tracking errors.
Similarly, in U.S. Pat. No. 5,168,184, Teruo Umehara et al. disclose a swing-actuator assembly that uses a plastic molded hold member no thicker than the coil element to connect coil and block, thereby reducing rotational inertia without losing the desired rigidity of the coil-block joint. In U.S. Pat. No. 5,168,185, Umehara et al. address the related problem of disk drive contamination caused by "flash formation" during encapsulation (over-molding) of the actuator arm assembly. Umehara et al. show how to use a diluted epoxy coating over the encapsulated swing-type actuator to prevent shedding of injection-mold plastic particles (flashes). Takahashi et al. and Umehara et al. consider only the coil-block joint rigidity issues relating to coil 22 and E-block 12 in FIG. 1.
In U.S. Pat. No. 5,382,851, Robert Loubier discloses a different swing-type actuator of the type exemplified by the actuator 26 in FIG. 2. Actuator 26 reduces rotational inertia by encapsulating a coil carrier 30 and individual metallic actuator arms exemplified by the actuator arm 28 into a central plastic body 32, thereby eliminating the heavy central body 46 (FIG. 1) of the E-block known in the art. Loubier's invention introduces several new disadvantages. The coil-block joint problem known in the art is exacerbated because most of the central portion 46 of the E-block is replaced with plastic, thereby perhaps introducing more flexibility in the alignment between coil carrier 22 and actuator arm plurality 48. Also, pivot-axis runout is increased because of increased flexibility at the inner surface of the bore hole 40. Loubier does not consider solutions to this problem beyond merely drilling into plastic pivot body 32 a bore hole 40 to accept an unspecified cartridge bearing assembly. Loubier considers no means for rigidly attaching his cartridge bearing assembly (not shown) to his molded plastic body 32.
The introduction into non-conductive plastic body 32 of individual conductive actuator arms exemplified by actuator arm 28 creates a static charge problem that is not known for monolithic metallic E-block actuator assemblies. Loubier resolves this problem by using a press-fit conductive pin 42 inserted through a series of precisely-aligned holes in the actuator arm plurality 48, as shown in FIG. 3. One end of conductive pin 42 is then coupled to a ground potential in some manner. The precise alignment of the actuator arm plurality 48 needed for insertion of grounding pin 42 requires jigging or drilling steps additional to those steps required for actuator fabrication using a monolithic aluminum block. He also suggests coupling arm plurality 48 through conductive pivot journal 38 at the edges exemplified by edge 44 (FIG. 3), or by means of a conductive plastic filler (not shown) contacting actuator arm plurality 48 in body 32. Although each of these techniques resolves the static charge accumulation problem, all introduce some particular new fabrication steps, thereby increasing manufacturing cost. Other examples of prior art actuators include those shown in U.S. Pat. No. 4,985,652 to Oudet, et al. and U.S. Pat. No. 4,916,456 to Manzke, et al.
In some swing-type actuator embodiments, the arm is constructed from metal such as aluminum. Metal arms have several advantages, including the ability to more readily and securely attach read/write heads to the arm, the ability to maintain exacting tolerances for the arm, and the relative ability to more easily machine features into the arm. The coil is typically attached to metal arms by means of screws, similar fasteners or adhesives. However, for various reasons, including ease of manufacturing, it has been found desirable to secure the coil to the arm by injection molding thermoplastic material around a portion of the coil and a portion of the arm. Examples of such actuators include U.S. Pat. No. 5,122,703 to Takahashi, al., as well as numerous earlier actuators manufactured by the assignee of the present application.
Although the above-described actuators are all acceptable for their intended uses, they do have certain undesirable features. For example, an aluminum E-block can be relatively expensive to manufacture. Additionally, it may be desirable to provide an actuator having a lower overall weight than one utilizing an aluminum E-block, thereby reducing inertia and momentum and the attendant forces necessary to move and stop the actuator. Accordingly, all-plastic actuators have been proposed. In such actuators, the coil is placed in a mold and the actuator arm is simultaneously formed from a thermoplastic material and molded to the coil. Examples of such actuators include U.S. Pat. No. 5,165,090 to Takahashi, et al., as well as earlier actuators manufactured by the assignee of the present application.
Embodiments of the invention in U.S. Pat. No. 5,654,844 solve two of the previously stated problems. The static accumulation problem is resolved by adding a spring-loaded grounding element that eliminates all fabrication steps associated with a conductive filler, a molded metallic journal or a carefully-aligned press-fit conductive element. This design ensures rigid coupling of the plastic actuator body to a pivot axis without fasteners or metallic journals molded into the plastic body. The spring-loaded ground conductor element adds part cost and assembly complexity. The press-fit pivot element of this design may degrade E-block journal dimensional accuracy during manufacturing rework steps where a defective first pivot is pressed out and replaced by a second pivot into a now deformed journal. This could degrade actuator tracking performance and reliability while increasing plastic actuator manufacturing cost.
Without solutions for these and other disadvantages of the molded plastic actuator assembly know in the art, practitioners are obliged to accept unwanted new fabrication costs to obtain a desired reduction in rotational inertia. Certain of these unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.